1. Field of the Invention
This invention relates to a semiconductor avalanche photo-diode (referred to hereinafter as APD). More particularly, it relates to an improvement of the APD in dark current, noise and high frequency characteristics by achieving uniform avalanche multiplication therein.
2. Description of the Related Art
A long-distance and high-capacity optical transmission using a wave length of 1.0 to 1.7 .mu.m, where the optical transmission loss of a silica optical fiber becomes minimum, has been increasing in practical use. As an optical detector for this application, an APD device composed of InP/GaInAs (Indium-Phosphorus/Galium-Indium-Arsenide) has been generally used. Its device structure is schematically shown by a cross-sectional side view in FIG. 1. A DC operating voltage of approximately 100 V is applied between a negative electrode 18 and a positive electrode 19. The numeral 11 denotes an n.sup.+ -type InP substrate. The numeral 12 denotes a light-absorbing layer made of n-type GaInAs, in which electrons and positive holes are excited by an injected light thereinto. The mark "e" denotes an electron, and the mark "ph" denotes a positive hole both of which are produced by light excitation. The arrow marks indicate directions in which the electrons and positive holes move when an operating voltage is applied. The numeral 14 denotes a window layer, or sometimes called a multiplication region, made of InP, in which the positive holes from the light-absorbing region 12 are accelerated by the externally applied voltage so that an avalanche multiplication takes place therein. The numeral 16' denotes a guardring-burying layer made of n.sup.- -type InP. The numeral 15 denotes a p.sup.+ -type region doped in the window layer 14, with which a pn junction 17 is formed. The p.sup.+ -type region 15 and the window layer 14 are transparent so that the injected light can reach the light-absorbing layer 12. The numeral 16 denotes a p-type guardring formed in a guardring-burying layer 16' for relaxing excessive concentration of electric field lines which may cause avalanche breakdown at unexpected low voltage.
However, on the other hand, in order to reduce a multiplication noise and dark current produced in the device of FIG. 1, an APD device employing a resonant impact ionization, which was reported by O. Hildebrand et al. in IEEE JQE-17, No 2, page 284, 1981, has been proposed by T. Kaneda, one of the present inventors, as disclosed in the provisional publication of the Japanese patent Sho No. 60-105281, structure of which is schematically shown by the cross-sectional side view in FIG. 2, or by P. Pichard et al., "1.3 .mu.m CdHgTe AVALANCHE PHOTODIODES FOR FIBER OPTIC APPLICATION" at page 479 of 9th European Conference on Optical Communication, 1983. In FIG. 2, the numeral 21 denotes a p.sup.+ -type GaSb (Galium-Antimony) substrate. The numeral 24 denotes a window layer made of p-type Ga.sub.0.6 Al.sub.3.4 Sb (Galium-Aluminum whose contents ratio is 0.6:0.4, and Antimony). In the window layer 24, an n.sup.+ region 25 is provided to form a pn junction 26 with the window layer 24. The numeral 27 denotes a protection film. The numeral 28 denotes an anti-reflection layer. The anti-reflection layer 28, the n.sup.+ region 25 and the window layer 24 are all transparent for an injected light which is introduced into a light-absorbing layer 22 described below. The light-absorbing layer 22 is made of p-type Ga.sub.0.9 Al.sub.0.1 Sb, were electrons and positive holes are produced by an excitation of the injected light thereinto. The numeral 23 denotes an avalanche multiplication region made of Ga.sub.0.935 Al.sub.0.065 Sb. This particular material, Ga.sub.0.935 Al.sub.0.065 Sb, can cause an avalanche multiplication by the resonant impact ionization due to positive holes from the light absorbing layer 22 when an electric field of approximately 4.times.10.sup.4 V/cm is applied thereto. This requires approximately 20 V between the electrode 20 acting as a positive terminal and an electrode 29 acting as a negative terminal.
In the APD configuration of FIG. 2, the alvalanche multiplication layer 23 must be located on the opposite side of the light absorbing layer 22 from the pn junction 26. In other words, the layers are arranged in the order of: the pn junction 26, window layer 24, the light absorbing layer 22 and the avalanche multiplication layer 23. Accordingly, when the operating reverse voltage is applied between the electrodes 29 and 30 in order to obtain a necessary electric field intensity, i.e. 40.times.10.sup.4 V/cm, in the avalanche multiplication layer 23, distribution of the electric field intensity is as shown by the curve labelled "prior art" in FIG. 5. Consequently, the applied voltage corresponding to the required field intensity must be larger than a certain value, and a greater electric field than that in the avalanche multiplication layer 23 must be applied also to the light absorbing layer 22. Thus, there arises undesirable problems that a dark current due to a tunnel effect remains. It is also difficult to dope n-type impurities into the p-type GaAlSb window layer in a mass production process. Therefore, an easy structure for fabrication of the APD employing resonant impact ionization has been requested.